New research shows that one in three women are being wrongly diagnosed with breast cancer that requires treatment. Photograph: Martin Godwin

One in three women who is told she has breast cancer after screening is being diagnosed and treated unnecessarily, scientists say today.

Not all breast cancers are potential killers, say researchers in a paper in today's British Medical Journal. Some are inconsequential. If they were not picked up, women would not know they had them. But because they are detected through breast cancer screening, women usually undergo surgery and chemotherapy which are traumatic and potentially harmful.

The Nordic Cochrane Centre group, which did the research, has identified over-diagnosis of breast cancer in the past from the original trials carried out before mammography screening was widely introduced. But in today's paper, it calculates the extent of that over-diagnosis (detecting harmless cancers) in real populations where screening is offered in the UK, Canada, Australia, Sweden and Norway.

It is no longer contested that screening leads to over-diagnosis, according to an editorial published by the BMJ. "The question is no longer whether, but how often, it occurs," writes Gilbert Welch, professor of medicine at the Dartmouth Institute for Health Policy and Clinical Practice in the US. "The NHS recently scrapped its leaflet inviting women to undergo mammography in response to criticisms that it failed to mention the major harm of screening – over-diagnosis."

Some cancers grow so slowly that the person eventually dies of something else, while others are dormant or even regress, he says.

"Because doctors don't know which patients are over-diagnosed, we tend to treat them all. Over-diagnosis therefore results in unnecessary treatment.

With the advent of widespread efforts to diagnose cancer earlier, over-diagnosis has become an increasingly vexing problem."

In other cancers, it is well recognised that there is a risk of picking up and treating tumours that would have done no harm. Prostate cancer is an obvious example, where the advice to men in the UK who have a screening test (although it is far from conclusive) is to watch and wait. But neuroblastoma, melanoma, thyroid cancer and lung cancer can also sometimes be detected and yet cause no harm.

"Mammography is one of medicine's 'close calls' – a delicate balance between benefit and harm – where different people in the same situation might reasonably make different choices. Mammography undoubtedly helps some women but hurts others. No right answer exists, instead it is a personal choice," writes Professor Welch.

The study, by Karsten Jorgensen and Peter Gotzsche, looked at breast cancer trends seven years before and seven years after screening was introduced in the five countries. They also took account of other factors that may have affected the results, such as changes in background levels of breast cancer and any compensatory drop in rates of breast cancer among older, previously screened women.